In general, Radio Resource Management (RRM) is very important in wireless communication systems.
RRM is at the heart of any advanced wireless system and determines how the radio resources are utilized. For example, which radio channel, transmission power, transmission mode, modulation and coding that should be used and which base station/access point the user should be connected to.
The RRM solution to a large extent determines the performance of the wireless network.
Modern wireless communication systems, such as IEEE 802.11ay and 3GPP 5G, also referred to as New Radio (NR), operating on high frequencies are dependent on advanced antenna solutions with beam forming and beam tracking which is also part of RRM.
Such systems will use advanced antenna systems containing large antenna arrays for data transmission. With large antenna arrays, data signals will be transmitted in narrow beams to increase signal strength in some directions, and/or to reduce interference in other directions. On one hand, this is done to obtain improved link quality and to enable spatial separation and reduce interference between users. On the other hand, using arrays is necessary to ensure sufficient link quality in high-frequency deployments where the individual antenna element apertures are small and do not capture sufficient signal energy individually. By coherently aligning the elements, gives rise to effective beam gain, but also beam directivity in a certain direction.
FIG. 1 is a schematic diagram illustrating an example of a wireless communication system operating using directive beams for serving wireless user devices. The wireless communication system 100 may be based on a radio access part including network nodes 110 such as radio access network nodes (e.g. base stations and/or access points), which operate based on directive beams for serving one or more wireless user devices 120 such as User Equipment (UE). The wireless communication system may include and/or be connected to a core network (not shown) and/or other network parts responsible for network functions such as mobility management, connection to other networks and resource management.
In active mode, the connection of a moving UE must be seamlessly handed over as the UE moves across the different cell coverage areas in the network. Handover is the process of transferring an ongoing connection of a UE from one node (the serving) to another node (the target), or from one cell to another within the same node. This is done to accomplish a transparent service or service continuity over a larger area. The handover should happen without any loss of data and preferably with no interruption.
In legacy cell-based systems like LTE, the cell-specific reference signals (CRSs) have been used for mobility measurements. These are broadcasted in all neighbor cells in an always-on manner over the entire bandwidth, regardless of the presence or position of UEs in the system. The CRS are easy to measure and yield consistent results, but static CRS signaling leads to high resource usage, power consumption and constant inter-cell interference generation in the downlink.
All base stations continuously transmit pilot signals that UEs in own and neighbor cells use to estimate the target cell quality. This is also true in GSM (BCCH), WCDMA (CPICH) and in WiFi (beacon). Each UE performs periodic measurements and reports the measurement results to the network when certain reporting conditions are met (periodic or event based). If it is detected that the serving cell quality is getting close to another candidate's cell quality, a more detailed measurement process or a handover procedure may be initiated.
In some configurations, initial access signals and other associated signals like the Primary Synchronization Signal/Secondary Synchronization Signal (PSS/SSS), if transmitted at a sufficient rate, may also be used for active mode mobility measurements. They allow estimating the link qualities with respect to the candidate cells, for the purposes of measurement reporting back to the network.
In modern beam-based systems, serving and target node identities are often no longer sufficient for maintaining seamless connections during inter-node handover. Handover management between narrow beams in neighboring base stations becomes a necessity, and the serving base station also needs to decide if a beam switch or beam update is necessary within the own cell. The serving link may thus effectively be the beam through which the base station is currently communicating with the UE, and the beam it will hand over or switch to becomes the target link.
In a beam-based system like NR, it is desired to avoid excessive static downlink reference signal (RS) signaling, so instead the network may turn on special Mobility Reference Signals (MRS) only when needed, e.g. when there are UEs found in a given network region or, in a UE-specific manner only in relevant candidate beams. It may be done periodically or when the network determines that a beam update for the UE may be needed, e.g. when decreasing serving beam quality is detected. Each activated beam transmits an MRS that carries the beam identity.
In 3GPP, it has been agreed that Channel State Information Reference Signal (CSI-RS) like signal structures may be used as an MRS for mobility measurements, in addition to the PSS/SSS signals. The motivation for using CSI-RS may be e.g. of the following:                Inter-TRP (Transmission Reception Point) mobility in multi-TRP cells where PSS/SSS is transmitted.        Desired beam mobility resolution is higher than PSS/SSS beam sweep resolution.        Wide-band measurements desired for improved fading robustness in moderately dispersive environments or to improve measurement accuracy.        In contrast to PSS/SSS, CSI-RS can be dynamically turned on and off and configured according to suitable parameters (period, bandwidth, number of unique links supported, and so forth) by the network based on the presence of UEs and their mobility needs.        
To achieve high performance in a wireless network, there is a need for an efficient RRM solution. This will be increasingly important in the coming 5G deployments for several reasons. For efficient resource use, modern systems such as IEEE 802.11 ay and NR rely on advanced antenna solutions such as beam forming and beam tracking. In order to perform accurate beam tracking, extensive measurements and signaling are needed which is costly in terms of radio resources and energy consumption.
Another reason is that NR will operate on high frequencies where the radio signal is more sensitive, e.g. to blocking by persons, vehicles, buildings and other obstacles. The radio reception in a modern systems like IEEE 802.11 ay or NR operating in the mmWave band using a narrow beam can be lost completely if a person walks into the space between the transmitter and the receiver (indoors) or if a vehicle, such as a bus or a truck, drives between the transmitter and the receiver (outdoors). This will call for fast RRM algorithms that can make fast decisions to avoid the session drop/radio link failure proactively.